The homodyne (or direct conversion) receiver architecture is commonly used in wireless devices like mobile phones, enabling a low cost to be achieved. In this receiver architecture, the incoming radio frequency (RF) signal is converted to a baseband signal in a single frequency conversion step. This type of receiver is therefore relatively sensitive to low frequency disturbances generated by the mixer at its output. Some of these disturbances are due to mismatches in the mixer, like even order distortion and DC-offset, while another problem is low frequency flicker noise (1/f noise), generated in the mixer switches.
To avoid flicker noise, it is common to use passive mixers in homodyne receivers. Furthermore, quadrature down-conversion is normally used in homodyne receivers to keep the phase information of the signal. A passive mixer where mixer switches are driven by 25% duty cycle square wave is often used. The 25% duty cycle assures that just one mixer switch is closed at a time, minimizing harmful interaction within the mixer. The passive mixer also has the advantage of relatively low (or “almost zero”) power consumption in the mixer itself, but the local oscillator (LO) driver supplying the 25% duty cycle signals to the mixer switches instead consumes considerable power.
Mismatch in the switches causes even order distortion. The required second order input referred intercept point (IIP2) is relatively high in homodyne cellular phone receivers, e.g. with numbers in the order of +50 dBm referred to the antenna input. This makes it necessary to use relatively large mixer switches (e.g. large transistors) to achieve sufficient matching, resulting in a relatively high capacitive load of the LO-driver. Because the power consumption of the LO-driver is proportional to the capacitive load, this results in relatively high power consumption. This is unwanted, because low power consumption is often desirable in mobile phones and other wireless devices.